Acid N depositions in the Bohemian Forest during the second half of the last century caused enormous soil acidification which led to the leaching of essential nutrients including nitrates. We investigated the effect of dissolved organic matter (DOM) and pH on the abundance of 16S RDNA, nirK and nirS gene copies in four spruce forest sites. Soil samples for molecular based quantification (qPCR) were taken from the organic litter and humus layers. The amounts of dissolved organic carbon (DOC) and dissolved nitrogen (DN) were much lower in highly acidified soils. We found a strong correlation between nirK denitrifiers and the amount of available P (r = 0.83, p \ 0.001), which suggested a higher nutrient sensitivity of this group of denitrifying bacteria. Additionally, we found that correlations between the amount of nirK denitrifiers and DOC and pH are exponentional showing two important threshold values, being 4.8 mol kg -1 and 5, respectively. The amount of nirK denitrifiers rapidly decreased below these values. The amount of nirK and nirS denitrifiers was higher in the organic litter horizon than the organic humus horizon at all sampling sites.

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The role of litter composition and quality on the nutrient release was studied in three month laboratory experiment. Spruce needles and leaves of four species dominant in understorey vegetation of the Norway spruce forest were collected in early autumn and incubated at 5• C, 10• C and 15• C. C mineralization was measured every two weeks, concentration of NH4, NO3, dissolved organic N, dissolved organic C and oxalate extractable P at the beginning and end of incubation and decay rate and nutrient release was calculated. Freshly senescent leaves contained less N and P indicating nutrient reallocation. Effect of temperature on a decay rate and nutrient transformation was not significant while the effect of litter quality expressed by C/N ratio at the end of incubation was. The decay rate was the fastest for the fern (Athyrium alpestre) and decreased in order: Callamagrostis villosa > Vaccinium myrtillus > Avenella flexuosa > spruce needles. The critical C/N ratio bellow which mineral N was released in high amount was around a value of 32. The results indicte that an increase of coverage of understorey vegetation can increase a risk of nutrient release.

We conducted over four months a shortterm laboratory incubation experiment to find the best prediction parameters (i.e. initial chemical characteristics) to explain differences in microbial respiration rates and mineral N (DIN) release in different litter in an acidified spruce forest. In addition, we wanted to find the link between the activity of key extracellular ligninolytic enzymes, phenoloxidases (PhOx) and peroxidases (Perox), microbial respiration and composition of fungal and bacterial communities. Samples of spruce needles (Picea abies) and litter of four dominant understorey vegetation; lady fern (Athyrium alpestre), blueberry (Vaccinium myrtillus), reedgrass (Calamagrostis villosa) and hair grass (Avenella flexuosa), were collected in 2005, 2006 and 2007 from six sites located in watersheds of two glacial lakes (Plesne Lake and Certovo Lake) in the Bohemian Forest, Czech Republic. Litter samples were incubated at 0 and 10°C in laboratory controlled conditions for 90 days. Activities of PhOx and PerOx, and C mineralization rate were measured regularly each 14 days. Litter quality characteristics and endophytic microbial community structure, based on 16SrDNA-DGGE fingerprint of bacteria and ITS-DGGE of fungi, were determined at the beginning and end of litter incubation. Our results showed a close correlation of phenolics/P OX with DIN release (r [ 0.74, p \ 0.001). Using multivariate analyses, P OX seems to play an important role in the change of litter fungal and bacterial community composition. At 0°C the fungal and bacterial communities of reedgrass and blueberry litter changed in relation to P OX and Perox activity, while at 10°C the fungal communities after the incubation were additionally affected by the phenolics/N TOT and phenolics/P TOT ratios.